1. Field of the Invention
The present invention relates to a slide fastener stringer including a stringer tape supporting thereon coupling elements each having a pair of upper and lower legs sandwiching therebetween a longitudinal edge of the stringer tape.
2. Prior Art
One of the most widely used stitch types is multi-thread chain stitches or "double locked stitches" that include needle and looper threads both having loops that are interlaced and interlooped. Such stitch type has found particular utility for securing slide fastener coupling elements to a stringer tape because the needle and looper threads can be supplied from large-capacity bobbins for continuous sewing operation. It is especially advantageous to mount helically coiled coupling elements on a stringer tape by utilizing the double locked stitches since the loops of the looper thread engage those legs of the elements which are remote from the stringer tape, and are less stretchable than the needle thread. Slide fastener stringers having coiled fastener elements thus fastened are highly resistant to sharp bending, and hence can be coupled with mating stringers with an increased degree of strength against accidental separation.
However, when the double locked stitches are employed to fasten to a stringer tape coupling elements each having a pair of upper and lower legs disposed one on each side of the tape, as shown in FIG. 4 of the accompanying drawings, the fastener stringer can be sharply bent in one direction as a portion of the needle thread which is disposed on one or lower side of the tape has no interlaced or interlooped loops that give a resistance to such bending. Slide fasteners with such stringers intermeshed are liable to split open when subject to bending forces. To eliminate such a difficulty, it has been attempted to include a pair of additional threads interlaced with the needle thread over the element legs on the lower side of the tape as illustrated in FIG. 5 of the accompanying drawings. Addition of such reinforcing threads however is costly and involves complicated sewing operation which must be accomplished by a specially designed sewing machine.